fix(material/tree): not rendering initial data from flat data source

[crisbeto]

The MatTreeFlatDataSource has an initialData parameter which doesn't work, because it only assigns the data to one of the three streams used to render the tree.

These changes also make some underscored properties private.

Fixes #22282.

[wagnermaciel]

_flattenedData is used internally in ~10 files, so making it private is a breaking change

@andrewseguin what is the standard response to people using fake private props? Isn't this discouraged & not supported?

[crisbeto]

I've made it public again. Usually the underscore prefix means that it's private so it should discourage people from using it, but there's nothing actually stopping them from doing it.

[andrewseguin]

Yeah...they should not have been using it but unless we put private then we can't do much about it. If we want, we can modify their code based on what they are trying to achieve. Also, if their uses are legit, maybe we consider making it public (without underscore)

[wagnermaciel]

@andrewseguin My thoughts exactly. I know literally nothing about the angular material tree, but is there another way to access the _flattenedData other than the property? Also, I've sent you a few links to some places where _flattenedData is being used in g3

[wagnermaciel]

@crisbeto I think we should be good to land this internally but there are some merge conflicts that need to be resolved before I can run presubmits

[crisbeto]

Done.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}